POTASSIUM-DEPENDENT CHANGES IN THE EXPRESSION OF MEMBRANE-ASSOCIATED PROTEINS IN BARLEY ROOTS .1. CORRELATIONS WITH K+(86RB+) INFLUX AND ROOT K+ CONCENTRATION

Barley (Hordeum vulgare L. cv Halcyon) seedlings which had been grown in full strength complete inorganic nutrient media (containing 6 millimolar K+) had high internal K+ concentrations and low values of K+ (86Rb+) influx when influx was measured from solutions containing 100 micromolar K+. Transfer of these plants to solutions lacking K+ resulted in significant reductions of root and shoot K+ concentrations and values of K+ (86Rb+) influx increased by greater than 10-fold within 3 days. When plants treated in this way were returned to complete solutions, containing K+, the changes induced by K+ deprivation were reversed. Parallel studies of microsomal membranes by means of SDS-PAGE demonstrated that the expression of a group of polypeptides increased or decreased in parallel with changes of K+ (86Rb+) influx. Most prominent of these were 45 and 34 kilodalton polypeptides which specifically responded to K+ status of the barley plants; their expression was not enhanced by N or P deprivation. The 45 kilodalton polypeptide was susceptible to degradation by a membrane associated protease when microsomes were washed in buffer containing 0.2 millimolar PMSF. This loss was prevented by increasing PMSF concentration to 2 millimolar.